Poly Tactics adopted by Bajaj financial company

I had borrowed a “Cross sell personal loan - from Bajaj Finance ltd on May 2012, as per my loan documents and welcome letter it was clearly stated that interest of rate on the above-said loan was 19.99%- fixed for 36 months. But as per A/c statement, Bajaj charged the rate of interest @ 33.59% reducing through Amortization calculation. Since, I have closed my loan account after payment of 21 regular EMI’s @ 4755/-. I should be charged the promised fixed rate of interest i.e. 19.99% only.
My total payment till closing of my account comes to Rs. 144431/- where as I have been chargeRRs.158476/- instead.
Can an Bajaj (NBFC) charge interest @33.59%(reducing) for a personal loan given on 19.99% flat rate of interest , when pre-closed after 21 months?????